1. Field of the Invention
The invention relates to linear battery chargers.
2. Description of the Related Art
Conventional linear battery chargers can be operated in constant current and constant voltage charge modes. Before reaching a rated voltage level, a battery module is charged by a constant current provided by a linear battery charger operating in the constant current charge mode. When the voltage level of the battery module (Vbat) reaches the rated voltage level, the linear battery charger switches to the constant voltage charge mode to charge the battery module to a desired voltage level. For example, to charge a lithium battery having a desired voltage level of 4.2V, the linear battery charger begins operating in a constant current charge mode to charge the lithium battery to a rated voltage level of 4.1V and then switches to a constant voltage charge mode to charge the lithium battery to the desired voltage level, 4.2V. When the linear battery charger is operated in the constant voltage charge mode, the lithium battery is kept from overcharging, and the lifespan thereof extended accordingly.
FIG. 1 illustrates a conventional linear battery charger 100, comprising a charge transistor gmp charging a battery module 102. In a constant current charge mode, a first switch SW1 is turned on and a second switch SW2 is turned off. The loop comprising a first operational amplifier OP1, a second operational amplifier OP2, a transistor gmnc and the charge transistor gmp maintains current I through the current-sensing resistor Rsns at a constant current level. The value of the constant current level is determined by a first reference voltage level Vref1 coupled to the second operational amplifier OP2. The linear battery module 100 further comprises a frequency compensation circuit comprising a capacitor CC and a resistor RC to compensate the undesired pole of the linear battery charger in the constant current charge mode.
In a constant voltage charge mode, the first switch SW1 is turned off and a second switch SW2 is turned on. The loop comprising a third operational amplifier OP3, a transistor gmnv and the charge transistor gmp maintains the voltage level of the battery module (Vbat) at a second reference voltage Vref2. The linear battery module 100 further comprises a frequency compensation circuit comprising a capacitor CZ and a resistor RZ to compensate the undesired pole of the linear battery charger in the constant voltage charge mode.
As shown in FIG. 1, to ensure the conventional linear battery charger 100 operates normally in the constant current and constant voltage charge modes, the operational amplifiers OP1, OP2, and OP3 have to be accurate operational amplifiers (having low offset). The cost of the conventional linear battery charger 100 is very high. Furthermore, the frequency compensation circuits, especially the frequency compensation circuit comprising the capacitor CZ and resistor RZ, is very large and occupies considerable chip area.
Linear batteries that address such shortcomings are thus called for.